Electron emission effects from the solid surfaces include: (1) thermionic emission in which electrons are emitted due to heat, and (2) field electron emission in which electrons are emitted due to electric field. In recent years, cold cathode emitters of a field emission type (FE emitters) which do not require heating have attracted attention. For example, Spindt-type and thin-film emitters are known as such FE emitters.
Spindt-type electron-emitting elements are the basic type of FE emitters. The operation thereof is based on causing the emission of electrons in vacuum by applying a high electric field (>1×109 V/m) to a distal end region of a fine conical emitter tip formed from a metal material with a high melting point, such as silicon (Si) and molybdenum (Mo) (see, for example, U.S. Pat. No. 3,665,241).
A thin-film electron-emitting element represents the development of the Spindt-type electron-emitting element. Thin-film electron-emitting elements do not use the fine conical structure typical for the Spindt-type elements and electrons are emitted from a flat emitter. In this element, because the emitter has a flat shape, an electric field concentration effect obtained with the conical structure cannot be expected. For this reason, a limitation is placed on the emitter materials that can be used in the thin-film electron-emitting elements.
Carbon materials such as amorphous carbon films, diamond, and carbon nanotubes (CNT) are known as materials for emitters (for example, see, Japanese Unexamined Patent Publication Nos. H8-505259, H7-282715, and H10-012124). Among the aforementioned carbon materials, CNT is a fine tubular material (diameter in the order of several to several tens of nanometer) having a shape in which a graphen sheet composed only of carbon is wound into a cylinder. This material is electrically conductive and has a sharp form with a large aspect ratio. For this reason, it is the most promising as an effective emitter material among all the carbon materials.
Furthermore, a field electron-emitting element is known which comprises an emitter electrode for emitting electrons under applied electric field, an electron accelerating layer, and an extraction electrode, wherein the electron accelerating layer is composed of a porous silica film (Japanese Unexamined Patent Publication No. 2000-285797). A material obtained by precipitating graphite or silicon inside pores is used as the porous silica film, but it is assumed that it will be disposed in contact with the extraction electrode.